Method for applying a carbon coating to optical fibers in aluminum reactor vessel

ABSTRACT

A method for applying a carbon coating to an optical fiber wherein the build up of reaction by-products within the reactor is reduced by providing cooler reactors walls, dual fiber exit ports with different inside diameters, and gas shielding at one fiber exit port.

This application is a divisional patent application of U.S. Ser. No.07/950,072 filed Sep. 23, 1992 and issued as U.S. Pat. No. 5,346,520;and Ser. No. 08/258,770 filed Jun. 13, 1994, abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus for providing a carbon-containingcoating on optical waveguide fibers.

Optical waveguide fibers are typically provided with abrasion-resistantcoatings such as silicone or polyurethane acrylate, for example. Thesecoatings are usually applied to the pristine surface of the fiber duringthe fiber drawing process. While these coatings provide protection fromabrasion, they do not provide adequate protection from corrosion orhydrogen attack.

Various chemicals, including water, can attack a fiber, affecting bothoptical and mechanical properties thereof. Microcracks in a fibersurface are regions which are more susceptible to such attack,especially when the fiber is under stress. The growth of thesemicrocracks due to chemical attack reduces the mechanical strength of afiber and may result in static fatigue or sudden failure of the fiber.

If a fiber is exposed to an atmosphere containing hydrogen, the hydrogenwill diffuse into the fiber. Such diffusion is detrimental to theoptical performance of the fiber. An attenuation increase caused by thediffusion of hydrogen after an optical fiber has been installed mayresult in degradation of the transmission link which includes the fiber.

The presence of water and hydrogen are of particular concern in opticalfiber applications such as underwater cables. These applications oftenrequire long-lengths between signal amplification, and there is littleor no tolerance for increased attenuation in the fibers during theirfiber service life. Also, replacement of fibers which have failed due tochemical attack would be prohibitively expensive.

Various coatings have been developed to provide protection to opticalwaveguide fibers from chemical attack and to increase the strength ofthe fiber. These coatings have contained various materials, includingcarbon, metals and ceramics. See, for example, U.S. Pat. No. 4,512,629(carbon coating); U.S. Pat. No. 4,592,932 (metallic coating); U.S. Pat.No. 4,118,211 (ceramic coating).

Metallic and ceramic coatings have been used with varying degrees ofsuccess with respect to the reduction of strength degradation due tomicrocracks in the fiber surface. However, such coatings have not provento be sufficiently impermeable to hydrogen.

Carbon coatings are known to produce water resistant, high strengthoptical fibers. See, for example, Kao et al. U.S. Pat. No. 4,183,621.Carbon coatings have also been shown to be sufficiently impermeable tohydrogen diffusion. Lemaire et al., "Hydrogen Permeation in OpticalFibres with Hermetic Carbon Coatings", Electronics Letters, vol 24, no.21, pages 1323-24, Oct. 13, 1988; Lu et al., "Recent Developments inHermetically Coated Optical Fiber", J. of Lightwave Technology, vol. 6,no. 2, pages 240-244, February, 1988; Lu et al., "Hermetically CoatedOptical Fibers", International Wire & Cable Symposium Proceedings, pages241-244, 1987.

One method for providing an optical waveguide fiber involves exposing afiber to a carbon-containing reactant gas and decomposing the reactantgas by heating it. The required heat for the reaction may be provided bythe temperature of the fiber itself, external heating means, or by somecombination of the two. The decomposition of the reactant gas produces ahigh molecular weight reaction product, which forms the desired carbonlayer on the fiber, and reaction by-products. The reaction by-productscan be either high molecular weight dry particulate matter similar tocarbon black, or low molecular weight oily droplets which solidify to agummy or glassy material. Also, some portion of the reactant gas mayremain unreacted.

Low molecular weight reaction by-products are preferentially formed atlower temperatures. High molecular weight reaction by-products arerapidly formed at temperatures above about 150° C. These high molecularweight reaction by-products can be formed as reactant gas flows alongwith the fiber through the reactor and is exposed to temperatures in thevicinity of the fiber which are higher than 150° C.

Prior art processes and devices for manufacturing optical fibers withcarbon coatings have encountered various problems. Alignment of thereactor, in which the carbon coating is applied to a fiber, with theother components of the fiber drawing apparatus is necessary for processstability and repeatability. Prior art devices which have disclosed thematerial used in construction of the reactor have disclosed the use ofquartz or silica tubes. See, for example, Oohashi et al. U.S. Pat. No.5,037,464; Ishiguro et al. European Patent Publication No. 0,374,926;Schultz et al. U.S. Pat. No. 4,735,856; Evans et al. UK PatentApplication No. 2,156,858. Due to the complex shapes required in thereactor design, fabrication of the reactor using quartz or silica lacksdimensional repeatability which adversely affects the ability toinitially align the reactor on the fiber drawing apparatus and tomaintain alignment during the fiber drawing process.

Additionally, quartz or silica reactors provide insulating properties,elevating the temperature within the reactor and accelerating theformation of undesired reaction by-products. As a result, the rate offormation of high molecular weight reaction by-products is greater thanthe formation of low molecular weight reaction by-products. Highmolecular weight reaction by-products may build up within the reactorand impinge on the fiber, damaging the carbon coating and possibly thefiber itself. Because these reaction by-products build up over time, theability to produce long lengths of coated optical fiber is reduced. Insome cases, entire production lots of greater than 100 km of fiber arerejected to ensure the quality of the coating if build up of highmolecular weight reaction by-products is detected within the reactor.

Bennett et al. U.S. Pat. No. 5,152,817, to be issued on Oct. 6, 1992, isassigned to the Assignee of the present application. Bennett et al.discloses an apparatus for providing long lengths of optical waveguidefiber with a carbon-containing coating without the build up of highmolecular weight reaction by-products within the reactor.

The apparatus of Bennett et al. is shown in FIG. 1. It consists of acombination of an upper isolation chamber 1, reaction chamber 2,receiving chamber 3, and lower isolation chamber 4. Fiber 5 enters theapparatus at fiber inlet 6 and exits through external fiber exit 7.Reactant gas is introduced at inlet 8. Reaction by-products areexhausted through external fiber exit port 7 or through outlet pipe 9which can be optionally provided. Shield gas is introduced to upperisolation chamber 1 and lower isolation chamber 4 through shield gasinlets 10 and 11 respectively. The inside diameter of external fiberexit port 7 is the same as the internal diameter of internal fiber exitport 13. In a preferred embodiment, the portion of reaction tube 14within chamber 2 may be perforated, as shown, to evenly distribute thereactant gas radially around fiber 5.

The length and diameter of reaction tube 14 of Bennett et al. areselected to ensure adequate coating thickness and to reduce the build upof high molecular weight reaction by-products inside the reactor. Thepreferred length is about 5-6.5 cm for an inside diameter (ID) of 1 cm.However, even with these preferred dimensions, oily reaction by-productmay build up on the interior surfaces of reaction tube 14. Bennett etal., col. 7, lines 2-6, Detailed Description. Reaction by-products willdeposit on the interior walls of receiving chamber 3. The insidediameter of receiving chamber 3 must be at least about 1 inch (2.5 cm)and the length at least 4 inches (10 cm) for the build up of reactionby-products on the walls of receiving chamber 3 to present no problem inthe fiber drawing or coating processes. Bennett et al., col. 7, lines16-21, Detailed Description.

Even with these design considerations, there is some build up ofreaction by-products at opening 12 of internal fiber exit port 13 inBennett et al. First, an oily low molecular weight reaction by-productwill deposit near opening 12. This oily film will solidify over time.High molecular weight particulate reaction by-products will then tend toadhere to the oily film. These particles will serve as sites at whichadditional reaction by-products will accumulate. This build up can causedamage to the carbon coating or the fiber itself. Even if this build updoes not apparently damage the coating or the fiber, detection of thebuild up will result in rejection of the fiber drawn to ensure thequality of the coating and the fiber. Therefore, even a slight build upis considered unacceptable.

In another embodiment, Bennett et al. discloses a reactor where thebottom surface of the receiving chamber is angled downwardly away fromopening 12 through which the fiber exits the receiving chamber. Thisangle is disclosed in a specific example as being 50° with respect tothe fiber axis. Bennett et al., col 10, lines 18-22, DetailedDescription. The purpose of this angled surface is to prevent any oilyreaction by-products which may deposit on the bottom surface of thereceiving chamber from flowing toward opening 12 before that depositsolidifies. Bennett et al, col. 10, lines 39-42, Detailed Description.

The reactor in Bennett et al. is made of glass (typically, "PYREX®")except for upper isolation chamber 1 and reaction chamber 2, which aremade of aluminum. After a preform is drawn into fiber, the reactor isremoved from the drawing apparatus and the glass portion is heated toabout 900° F. (480° C.) for a period of about four hours in anoxygen-containing atmosphere to burn off any reaction by-products whichmay have deposited on the surfaces of the reactor.

Glass reactors have been used instead of metal reactors for severalreasons. First, fiber drawing speeds are often less than 6 meters persecond. At these speeds, the temperature of the fiber may be reduced toa level at which the desired reaction will not occur unless the fiber iseither enclosed by an insulating material such as glass or some means ofauxiliary heat is provided. Second, during development, visual analysiswas required to determine any process parameter changes which wereneeded to stabilize the carbon coating reaction. Furthermore, becausealignment of the carbon coating apparatus was critical to the process,visual alignment was deemed necessary. These two requirementsnecessitated the use of glass reactors. Also, a metal reactor woulddeform at the temperature used for burning reaction by-products off thereactor walls as described above. If the burn off method is used with ametal reactor, it would require lower temperatures for unacceptablylonger periods of time than are used for burning off deposits on glassreactors. For example, an aluminum reactor exposed to an oxygenatmosphere at 750° F. (400° C.) for a period of ten hours stillexhibited some residue from the build up of reaction by-products.

Jochem European Patent Publication No. 0,393,755 discloses a method ofmanufacturing an optical fiber with a coating wherein the temperature ofthe reactor walls is below 800° C. The restriction on the maximumtemperature of the reactor walls is designed to reduce the build up ofreaction by-products on the reactor walls. The reactor "may comprise aninsulated wall or a heating device . . . in order to preclude that theglass fibre cools too rapidly." col. 4, lines 20-23. Jochem onlydiscloses reactor wall temperatures ranging from 600° to 900° C. in aspecific example. col. 7, lines 10-29. However, even at 600° C., webelieve significant amounts of reaction by-products will be deposited onthe reactor walls.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an apparatus for forming acarbon coating on long lengths of optical fiber. This object is achievedby using a reactor made of aluminum. This allows the reaction at or nearthe reactor walls to occur at temperatures which reduce the formation ofhigh molecular weight reaction by-products, and it provides a coolerreactor wall which retards the conversion of low molecular weightreaction by-products which are deposited on the reactor walls to highmolecular weight reaction by-products.

It is another object of this invention to provide a reactor in which thebuild up reaction by-products on the interior surfaces of the reactor isreduced by designing the reactor such that a boundary layer of reactionby-products and reaction gas which forms near or above the fiber surfaceis substantially undisturbed within the reactor. In another embodimentof the invention, the design of the fiber exit of the reactor utilizes adual fiber exit port with different port IDs which further reduces thebuild up of high molecular weight reaction by-products and subsequentdamage to the carbon coating or the fiber itself. In yet anotherembodiment of the invention, an inert gas shield is used to reduce buildup of high molecular weight reaction by-products at the fiber exit andto prevent oxygen from entering the reactor.

It is another object of this invention to provide a reactor design whichcan more easily withstand handling during use. This object is achievedby replacing the glass portions of the reactor with aluminum, or someother metal, which is less likely to break during cleaning or setupoperations.

It is another object of this invention to provide a reactor design withimproved dimensional repeatability by using metal in the manufacture ofthe reactor in place of glass.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a prior art apparatus for providing carboncoatings on optical fibers.

FIG. 2 is a cross-sectional representation of an apparatus in accordancewith the present invention.

FIG. 3 is a cross-sectional representation of a section of an apparatusincluding perforations in accordance with the present invention.

DETAILED DESCRIPTION

Equipment for the drawing of optical fiber from an optical fiber preformis well known in the art. A furnace is used to heat a preform to atemperature at which a fiber can be drawn therefrom. The diameter of thedrawn fiber is measured by a non-contact device. The fiber is thentypically coated with an abrasion-resistant coating and wound ontospools. See, for example, Jaeger et al., "Fiber Drawing and Control",Optical Fiber Telecommunications, chap. 9, pages 263-298, AcademicPress, 1979. If application of a carbon coating is desired, this isperformed after the fiber is drawn from the preform and before theabrasion-resistant coating is applied.

FIG. 2 shows one embodiment of the present invention. Only the carboncoating reactor is shown. The reactor 20 includes upper isolationchamber 21, reaction chamber 22, receiving chamber 23, and lowerisolation chamber 24. Reactant gas inlets 25 and 38 provide for theintroduction of reactant gas to reaction chamber 22.

Reactor 20 is preferably constructed of aluminum. The use of aluminumfor the reactor allows for tight tolerances (±0.005") in the manufactureof the reactor. These tighter manufacturing tolerances provide for morerepeatable alignment of the reactor with the other devices of the fiberdrawing apparatus. Also, with a metal reactor, any reactions of reactantgases at or near the reactor walls will occur at a cooler temperaturethan with the use of insulating materials such as quartz or silica.Because of the high thermal conductivity of aluminum, no externalcooling is required to provide the benefit of reduced reactortemperature.

These reduced temperatures allow the dominant reaction by-product at ornear the reactor walls to be low molecular weight oily droplets whichwill deposit on the reactor walls and form a film. To inhibit these lowmolecular weight reaction by-products from flowing toward the fiber,angled surface 35 is provided. These reaction by-products will form afilm on surface 35 and solidify. This build up does not impede the fiberdrawing and coating processes. The preferred angle is about 4.5°,although other angles may be used. Angles in the range of about 0°-15°have been tested. The minimum effective angle is about 2°, and webelieve that the maximum effective angle is about 67.5°.

Reactor 20 is positioned on the fiber drawing apparatus such that thetemperature of the fiber is sufficient to produce the desired reactionof the reactant gases. Fiber 26 enters reactor 20 at fiber inlet 33. Thedirection of fiber movement is shown by the arrow. The temperature offiber 26 at fiber inlet 33 is difficult to measure due to the small sizeof the fiber and drawing conditions, especially the draw speed. However,we estimate the temperature to be in the range of about 1,200°-1,800°C., with a minimum fiber temperature of about 1,000° C. being necessarywithin the reactor for the reaction to occur. Upper isolation chamber 21prevents the entry of ambient atmosphere into the top of the reactor.Upper isolation gas is introduced into upper isolation chamber 21through upper isolation gas inlets 34 and 39. In a preferred embodiment,tube 40 may be perforated, as shown in FIG. 3, to evenly distributeupper isolation gas radially about fiber 26. In a preferred embodiment,the perforations are in a spiralled pattern, i.e., the perforations in agiven row are not vertically aligned with the perforations in either therow immediately above or the row immediately below that given row.

Reactant gases are introduced through reactant gas inlets 25 and 38. Ina preferred embodiment, perforations are provided in the portion ofreaction tube 27 within chamber 22, as shown in FIG. 3, to evenlydistribute the reactant gas radially about fiber 26. In a preferredembodiment, the perforations are in a spiralled pattern, i.e., theperforations in a given row are not vertically aligned with theperforations in either the row immediately above or the row immediatelybelow that given row. The reactant gases react immediately uponcontacting fiber 26, producing the desired carbon coating. The reactioncontinues as the fiber moves through reaction chamber 27. A boundarylayer of high molecular weight reaction by-products, low molecularweight reaction by-products and unreacted gases forms near or above thesurface of the moving fiber. The reaction by-products and unreactedgases will accelerate to the fiber drawing speed and move with fiber 26as it passes through reactor 20. The diameter of internal fiber exitport 28 is chosen to avoid disruption of the boundary layer formed nearor above the fiber surface. This prevents high molecular weight reactionby-products contained in the boundary layer from building up aroundinternal fiber exit port 28. In one embodiment, the internal diameter ofinternal fiber exit port 28 is 0.51 inches (13 mm).

Build up of high molecular weight reaction by-products at external fiberexit port 29 is prevented by blanketing external fiber exit port 29 withshield gas introduced through shield gas inlets 30 and 31. The shieldgas may be any inert gas which keeps the reactor free of oxygen, withnitrogen being preferred. In a preferred embodiment, lower isolationtube 32, having a larger diameter than internal fiber exit port 28, isprovided, and lower isolation tube 32 is perforated to radiallydistribute the shield gas flow about fiber 26. These perforations arenot shown in the drawings, but they are similar to the perforations intube 40 and reaction tube 27, as shown in FIG. 3 and discussed above.The shield gas directs the boundary layer formed near or above the fibersurface out of the reactor through external fiber exit port 29, sweepingany high molecular weight reaction by-products out of the reactorthrough external fiber exit port 29. The shield gas also prevents theentry of ambient atmosphere into the reactor through external fiber exitport 29. In a preferred embodiment, the internal diameter of externalfiber exit port 29 is 0.255 inches (6 mm).

Reaction by-products contained in the boundary layer are exhaustedthrough external fiber exit port 29. These reaction by-products depositon glass tube 41, which is provided for that purpose. Glass tube 41 iscoated with a black glassy residue after a preform is drawn. Thisresidue does not interfere with the fiber drawing or coating processes.In a preferred embodiment, receiving chamber exhaust ports 36 and 37 areprovided to exhaust additional reaction by-products.

The reactor body may be made of any material which exhibits sufficientthermal conductivity to cool the reactor body. Means for auxiliarycooling of the reactor may optionally be provided if the material usedin construction of the reactor does not exhibit sufficient thermalconductivity to cool the reactor body without auxiliary cooling. Theauxiliary cooling may be passive (for example, fins attached to theexterior surfaces of the reactor body to increase the available surfacearea for convection cooling) or active (for example, a cooling jacketassembly attached to the exterior of the reactor body with cooling fluidcirculation). However, aluminum is preferred as it requires no auxiliarycooling and is easy to machine to tight tolerances. The temperature ofthe reactor walls is less than about 150° C. when aluminum is usedwithout any auxiliary cooling. At or below about 150° C., any reactionby-products which form at or near the walls of the reactor will have alow molecular weight and an oilier, more flowing consistency which willtend to cause these low molecular weight reaction by-products to depositon the reactor walls. Thus, substantially all of the high molecularweight reaction by-products which form within the reactor are entrainedwithin the boundary layer near or above the fiber surface and are notdeposited on the walls of the reactor as described above.

Some high molecular weight reaction by-products may escape the boundarylayer, particularly at points where the boundary layer is disrupted.Some disruption of the boundary layer occurs where reaction tube 27opens into receiving chamber 23. Any high molecular weight reactionby-products which escape the boundary layer at this point will stick tothe film formed on the receiving chamber walls by the low molecularweight reaction by-products. Additional low molecular weight reactionby-products will then tend to deposit over the high molecular weightreaction by-products. Because the receiving chamber is relatively largein diameter, as compared to the reaction tube and the internal andexternal fiber exit ports, any such build up does not interfere with thefiber drawing or coating process. Also, because of the relatively lowertemperature of the receiving chamber walls, as opposed to the highertemperatures at or near the fiber surface, the conversion of any buildup of low molecular weight reaction by-products on the receiving chamberwalls to high molecular weight reaction by-products is retarded. Thiswould be of particular concern at internal fiber exit port 28 orexternal fiber exit port 29, where the surfaces of those fiber exitports are in close proximity to the relatively high temperature of thefiber. Any low molecular weight reaction by-products which deposit overhigh molecular weight reaction by-products in these areas of the reactorwill tend to further react to form high molecular weight reactionby-products which may interfere with the fiber drawing and coatingprocesses. Therefore, it is important to prevent the disruption of theboundary layer in the vicinity of internal fiber exit port 28 andexternal fiber exit port 29, thereby preventing the deposition of anyhigh molecular weight reaction by-products in these areas of thereactor.

Shield gas may be provided at internal fiber exit port 28 by shield gasinlets not shown. Inert gases other than nitrogen may be used as theshield gas provided such other gases keep the reactor free of oxygen.Nitrogen is preferred because of its availability, ease in handling, andrelatively low cost.

The reactor is an assembly of smaller units. Upper isolation chamber 21is bolted to reaction chamber 22. Reaction chamber 22 is attached toreceiving chamber 23 via a rotating connection. Lower isolation chamber24 is attached to receiving chamber 23 using a similar connection. Theseconnections allow for quick disassembly of the reactor to facilitatecleaning, and quick reassembly of the reactor for reinstallation on thefiber drawing apparatus.

Test fibers were made using the present invention. The fibers were drawnfrom optical waveguide preforms which were produced using standardoutside vapor deposition (OVD) techniques as described in, for example,Berkey U.S. Pat. No. 4,453,961 and Berkey U.S. Pat. No. 4,486,212.However, any method suitable for producing preforms from which opticalfibers are drawn may be utilized. For example, vapor axial deposition(VAD) techniques, as described in Inada, "Recent Progress in FiberFabrication Techniques by Vapor-Phase Axial Deposition", IEEE J. ofQuantum Electronics, vol. QE-18, no. 10, October, 1982, and Suto et al.U.S. Pat. No. 4,367,085, may also be used to produce optical fiberpreforms. By running consecutive preforms without cleaning the reactorbetween preforms, greater than 300 km of optical fiber with a nominaldiameter of 125 μm have been produced using the present inventionwithout any significant build up of high molecular weight reactionby-products within the reactor.

Fiber was drawn from the preforms on standard optical fiber drawingequipment. The draw speed was 9 m/sec. The top of the reactor waslocated about 5.18 inches (13.2 cm) from the bottom of the draw furnace.Draw speeds in the range of about 7-9 m/see have been tested, and webelieve that speeds up to about 15 m/see are achievable using thepresent invention. The position of the reactor will vary based on thetype of fiber being coated, draw speed and other fiber drawing processparameters.

The reactant gas used was methyl acetylene at a flow rate of about 0.2liters per minute. Shield gas was provided only at upper isolationchamber 21 and lower isolation chamber 24 at a flow rate of about 2liters per minute for each chamber. Nitrogen was introduced throughupper isolation gas inlets 34 and 39 and shield gas inlets 30 and 31.

The reactor was visually inspected after each preform was drawn. Nosignificant build up of high molecular weight reaction by-products at ornear internal fiber exit port 28 or external fiber exit port 29 wasdetected in any of the tests. Build up of oily, low molecular weightreaction by-products was detected. This build up solidified and did notinterfere with the coating process.

Any reaction by-products which were deposited on the walls of thereactor were removed after the fiber drawing process was complete. Thisremoval was accomplished by directing a stream of powdered plastic(allypolycarbonate) at about 80 psi at the surfaces of the reactor. Thisprocess takes about 5 minutes. With prior reactors made of glass, theheating process used to remove reaction by-products deposited on reactorsurfaces takes about four hours at 900° F. (480° C.). The aluminumreactor of the present invention cannot be subjected to such hightemperatures without possible deformation of the reactor itself. In onetest, an aluminum reactor was heated to about 750° F. (400° C.) forabout ten hours in an oxygen atmosphere. There was still some of theresidue from the build up of reaction by-products on the reactor walls.The "blasting" technique described above is preferable as it is a vastimprovement in turn around time and effectiveness of cleaning over theheating process.

The fibers made under the above conditions were tested for hydrogenpermeation, strength, and fatigue.

The hydrogen permeation test is involves exposing fibers to purehydrogen at 11 atmospheres pressure and 85° C. for 21 days. 29 fibersfrom 10 different preforms were tested. The average attenuation increaseat 1240 nm was 0.006 dB/km for fibers made using the present invention.1240 nm represents the first harmonic of the fundamental hydrogenvibration and is used to characterize the level of hydrogen permeabilityof an optical fiber.

Fibers from the same 10 preforms were tested for strength and fatigueperformance. The average mean strength was 477 kpsi with a 1σ of 32.7kpsi. Strength was tested using 0.5 m gage lengths. The average Weibullslope was 73 with a 1σ of 11. The minimum fatigue constant, determinedby using FOTP-76, was 1130.

The present invention has been particularly shown and described withreference to the preferred embodiments thereof, however, it will be wellunderstood by those skilled in the art that various changes may be madein the form and details of these embodiments without departing from thetrue spirit and scope of the invention as defined by the followingclaims.

We claim:
 1. A method for applying a carbon coating to an opticalwaveguide fiber being drawn on a drawing apparatus, comprising the stepsof:(a) heating an end of a preform in a draw furnace, (b) drawing thefiber from said end of the preform, (c) introducing the fiber to areactor vessel comprising walls while supplying the reactor vessel witha reactant gas flow comprising a reactant gas, said reactant gascomprising carbon, (d) forming the carbon coating on the fiber bydecomposition of said reactant gas, wherein the reactor vessel walls arecomprised of aluminum.